This invention relates to novel metal coatings, which exhibit exceptional properties. More particularly this invention relates to metal coating compositions containing nickel, boron and particles and to the reductive deposition of said compositions on the surfaces of articles from alkaline aqueous solutions and the resulting articles.
The plating or deposition of metal alloys by chemical or electrochemical reduction of metal ions on the surface of an article to modify its surfaces characteristics for both decorative and functional purposes is well known in the art. Of particular commercial significance is the deposition of metal/metal alloy coatings on both metal and activated non-metal substrates to enhance surface properties such as hardness, resistance to corrosion, wear, and abrasive.
It is well known in the electroless metal plating art that solid particles can be co-deposited in nickel/phosphorus coating to change the properties of the coating. Particulate material such as diamond, silica carbide or Teflon or molly or tungsten disulfides have been used. The problem in the art is these co-deposited particles have poor bond strength and tend to separate from the nickel coating. This is thought to be due to the volume of particles lying at the interface of the nickel coating and the substrate thereby causing voids between the nickel coating and the substrate. This is can be seen under a cross section examination of the coating using a scanning electron microscope.
The prior art has recognized that when a borohydride reducing agent is substituted for a phosphorous reducing agent in a nickel plating bath a harder coating having greater wear resistance can be achieved. This is led to research and development effort in the area of nickel-boron coatings with the goal of producing still harder, more corrosion resistance coatings from a stable bath. For example, see, U.S. Pat. Nos. 6,066,400; 5,019,163; 4,833,041; 3,738,849; 3,674,447; 3,342,338; 3,378,400; 3,045,342; and 726,710. These references show conventional nickel/boron plating baths using conventional stabilizers. These disclosures are incorporated by reference.
In the development of nickel/boron coatings the prior art was faced with problem of stabilizing the bath due to the high reactivity of the borohydride reducing agent. The solution to the stability problem has been to add stabilizers such as thallium salts such as thallium sulfate, or lead chloride or lead tungstate to control the instability of the borohydride by slowing the reaction.
Controlling the stability of the borohydride requires balancing the need for a proper plating rate at the expense of stability by maintaining a proper amounts of stabilizer and boron reducing agent. To much stabilizer in the bath results in slower plating and the co-deposit of the stabilizer in the coating. An under stabilized bath tends cause the reaction to speed up causing seeding in the bath. Seeding is when the nickel plates out in the bath and forms small particles.
Also the balancing between the amounts of stabilizer and reducing agent must take in consideration the need to achieve about 3.5%-5.5% of baron in the coating to obtain optimum properties. Low baron causes softer coatings. To much boron causes the coating to be brittle.
The addition of stabilizers created new problems in the art by interfering with the formation of the nickel/boron coating. During the formation of the coating the stabilizer would co-deposit in the coating thereby negatively impacting the hardness of the coating.
Also as the bath ages there is a need to continuously add even additional stabilizers to achieve stability of the nickel/boron bath. During normal operation of the bath, boron and stabilizers are added every thirty minutes. In time the ability to achieve the proper amount of boron and nickel in the bath becomes very difficult. These baths usually become spend after 12 to 15 turnovers or less and have to be discarded. A turnover is when 100% of the available nickel in the bath is plated out and replenished. Although some of these baths could have a longer life, the cost of adding sufficient stabilizer to maintain the bath makes the plating uneconomical.
Another problem in the art was that many substrates such as such as certain aluminum alloys and tool steel alloys could not be heat treated without damage. To achieve the full hardness the nickel/boron coating should be heat treatment for ninety minutes at 725.degree. F. This heat treatment forms nickel boride crystalline clusters through out the coating. The coating's hardness and wear resistance are due to these crystals.
This invention solves these problems in the art of adding particles with the desired properties to a nickel bath using a borohydride reducing agent so that these particles are co-deposited with the nickel and boron. These particles impart to the coating the desired properties. By selecting specific particles of a specified size and type the properties of the coating can be enhanced. For example, hard particles provide better wear resistance. Lubricant particle such as molly disulfide provide lubricious properties.
The problem with the bond strength of the co deposited particles in a nickel coating when using a phosphorous reducing agent is not present when a borohydride is used as a reducing agent. Under the same SEM examination as above, a sodium borohydride reducing agent produced a layer of nickel boron coating 2-3 microns thick at the interface and then the particles distributed in the coating. The auto catalytic reduction of nickel in the presence of sodium borohydride is an almost an instantaneous reduction thereby producing a continuous layer of nickel boron at the interface without significant particle co-deposition. The absence of the particles in this interfacial layer is believed to account for the excellent bond strength in contrast to a layer having particles at he interface produced by a sodium hypophosphate reducing agent.
An object of this invention is to provide a dispersing composition that conditions the particles before adding to an electroless or electrochemical nickel/boron plating bath. The reason for the conditioning is to impart desirable properties to the particles to overcome the harmful effects when particles are added to the bath. The particles tend to induce seed out or plate out or fall out or negatively impact the plating rate. Seed out occurs when the nickel ions in the bath comes out of solution and acts as nucleation sites for nickel deposition. Plate out is when the nickel plates everywhere and the bath becomes unstable. Fall out is when nickel plated particles become larger debris particle and either falls to the bottom of the tank causing the further nickel plating or the debris particles falling on the work-item resulting in a rough, undesirable coating.
The conditioning allows the particles to stay in suspension in the bath by the normal agitation from the pump & filter. The tendency of the particles to clump or float to the surface in the bath is substantially reduced by providing agitation to the bath. Usually this accomplished by controlling the flow of the liquid in the bath. However, other mechanical devices can be used such as a propeller or by moving the holder for the substrates in the bath to provide agitation.